1. Field of the Invention
This invention relates to detection systems using either continuous wave or pulsed radio frequency energy to detect magnetic resonance responses, to receivers for such systems, and to corresponding methods and software for carrying out the methods, and to methods of using continuous wave or pulsed radio frequency energy to activate tags having a magnetic resonance response and to tags for such apparatus and methods.
2. Description of the Related Technology
Magnetic resonance Imaging (MRI) is a well-known technique to produce high quality tomographic images (2D slices or 3D images) inside the human body. MRI is based on the principles of nuclear magnetic resonance (NMR). MRI commonly uses DC magnetic fields of 1 Tesla (T) to 3 Tesla (T) to magnetize the nuclei. Electromagnetic waves in a narrow-band frequency range are selected to identify resonant properties (frequencies, relaxation times) of specific nuclei. For example, protons are commonly used for imaging of human organs due to the high concentration of H2O in the body with typical resonant frequencies close to 42 MHz at 1 T. Other nucleus Larmor frequencies are in the range 50 KHz-100 MHz and the entire frequency spectrum of interest in those imaging applications is of the order 10 kHz-1 MHz, which is an extremely narrow band, and the hardware is optimized towards generating and receiving this single-frequency e.g. a heterodyne detector matched to the frequency band of the source. In some MRI experiments a complex pulse sequence of narrowband RF waves is used to manipulate the phase of the nuclei so as to create a specific type of NMR signal. The pulse should rotate the spins of the nuclei over exactly 90 or 180 degrees and has to be tuned together with the nuclei resonance and can be 1-2000 microseconds.
In order for any tissue to be visible in an MRI image there must be contrast in the emitted signal (amplitude/phase/frequency) of the nuclei in the targeted and the adjacent tissue. This contrast can originate from differences in nuclear spin relaxation time, nucleus concentration or density. A contrast medium is often introduced into the body to enhance the contrast between the tissues by differential uptake. Paramagnetic or single domain (including super-paramagnetic particles) particles are often used as contrast agents that create oscillating fields when they tumble through a water environment, thereby changing the relaxation times (T1 and T2) of the tissues by introducing randomly fluctuating magnetic fields near the nucleus of interest, e.g. some tumor cells have a greater Gd uptake than the surrounding tissues, thereby decreasing T1 and resulting in strong contrast of tumors in MRI scans. Ferromagnetic resonance (FMR) imaging is a technique which is used for imaging ferromagnetic materials. In U.S. Pat. No. 6,924,150 a technique for narrow band radio frequency FMR imaging was proposed. Since the human body does not contain such properties by nature, FMR imaging was never considered for biomedicine applications. In a ferromagnetic material, the ferromagnetic resonance (FMR) signal is much stronger than NMR or other resonances due to the very high spin density and strong exchange coupling between the spins. The same exchange coupling also causes suppression of spin-spin relaxation such that the spin-lattice relaxation will dominate the total relaxation time.
Ferro- and ferri-magnetic nanoparticles are known for use as contrast agent in MR (Magnetic Resonance) imaging applications (at low frequencies) and for FMR imaging (U.S. Pat. No. 6,924,150).
In Nature 435 (2005) 1214, Gleich and Weizenecker describe a method of directly using magnetic particle imaging based on the nonlinearity of magnetization curves of ferromagnetic materials and the fact that the particle magnetization saturates at some magnetic field strength. The imaging techniques uses a time independent field that vanishes in the centre and increases in magnitude at the edges, such that only particles at that centre position are not saturated and respond to a second stimulus. Imaging is obtained by steering the central point of the time independent field through the volume of interest. The risks of using conventional MRI techniques are disclosed in “Magnetic Resonance Procedures: Health Effects and Society”, Ed. F. G. Shellock, CRC Press, 2001.
The use of magnetic medium to act as identification tags is already applied in magnetic RF-ID tags that consist of a magnetic medium which is detected when the article to which it is attached passes through a detection system, which emits an alternating narrow-band magnetic interrogation field of 50 Hz-100 KHz. Several patents (e.g. U.S. Pat. No. 4,940,966) discuss inventive magnetic bar coding or tagging principles based on distinctive physical parameters (e.g. shape, magnetic material, distance and orientation with respect to another tag). UWB radar technology is known for positioning of large articles (e.g. car identification, through wall vision) and motion sensing.
Magnetic RF-ID tags are known in U.S. Pat. No. 4,940,966 and implantable resonator circuits (LC tanks) including magnetic material in the inductor are shown in US2004138554 and US2005179552.
Magnetothermia (heating/destroying of cells) by specific uptake of nanoparticles and selective heating using AC magnetic fields below approx. 1 MHz is known. Heating of substrates by electromagnetic radiation at ferromagnetic resonance frequencies (1-300 GHz) is shown in US2004026028 and in [John Moreland, et al., Rev. of Sci. Instr., Vol 71 p 3088].